It is known in the prior art to refrigerate small enclosures by the use of thermoelectric modules. Typically, a thermoelectric module comprises a plurality of semi-conductors of the N-P type which are connected in series by a conductive material and thermally conductive plates. The semi-conductors are sandwiched between the plates and the current flow therein transfers heat from one plate to the other and this is well known as the Peltier effect. Accordingly, when current flows in the circuit, one of the plates is cold and the other is hot, thus the thermoelectric effect. A cold plate is actually another name for a cold side heat sink. Thermoelectric modules (TM) have a cold side and a hot side. The heat sink (or hot side heat sink) is mounted on the hot side of the TM, while the cold plate is mounted on the cold side of the TM.
One problem with the use of these thermoelectric modules is that as the temperature differential across the module becomes greater, its efficiency decreases. As efficiency is driven lower the cost of powering these devices increases and therefore these device have not been found applicable for use with large refrigeration units. The thermal stress, caused by large temperature differentials ΔT, across the module also damages the module. Further, because these thermoelectric modules are often connected in series with one another to provide ample heat transfer, thermal stress across these modules becomes very problematic if one of these modules or many become defective rendering the assembly inefficient and costly. Heretofore, inadequate solutions have been proposed to evacuate heat from the hot plate of the module at a rate sufficient to control temperature differential between the cold and hot plates of the module and these modules continue to be stressed by expansion and contraction which often cause excessive power consumption and cracking of the module.
Heretofore, in order to evacuate heat from the hot plate, heat sinks are secured to the hot plate and fans are used to circulate air through the heat sink. However, because ambient air outside a refrigeration unit using the thermoelectric module is often warm air it is not possible to significantly reduce the temperature differential across the thermoelectric module whereby to enhance the efficiency thereof. U.S. Patent Application 2006/0117761 A1, published on Jun. 8, 2006 and entitled “Thermoelectric Refrigeration System”, proposes an apparatus for thermoelectric cooling of an insulated enclosure using these modules and wherein heat pipes are used in conjunction with stacks of heat transfer fins whereby to more effectively transfer heat from the interior of the insulated enclosure and to also dissipate heat from the hot plate at the exterior of the enclosure. By the application of this heat pipe technology, the solution proposed in that publication is said to minimize thermoresistance across the thermoelectric modules. At both the hot and cold sides, the thermoelectric modules are joined to one face of a conductive copper plate and the heat pipes are joined to the opposite face thereof. The connecting copper plate is said to tend to balance loading of the module and of the heat pipes. The opposite ends of the heat pipes are joined into a stack of fins so as to provide adequate heat transfer area. This Publication also suggests ramping up and down the power supply to the thermoelectric modules whereby to reduce stress which is caused by pulse width modulated (PWM) current supplies where the supply is either ON or OFF inducing thermal shock in the module. The above-referenced Publication suggests the use of a variable power drive wherein the power supplied from 120 VAC source is connected to a power rectifier and a low voltage rectifier. The output from the low voltage rectifier supplies 12 VDC to a set point controller and a temperature sensor which responds to the temperature within the insulated enclosure. The set point and temperature DC signals are compared in a logic circuit and if the sensed temperature is more than 5 to 8° F. higher than the set point, a signal is sent to the power control device which ramps up the supply to full power over a period of 20 to 30 seconds from its initial level to maximum and this drive voltage is regulated by the logic circuit by decreasing the dry voltage proportionally to the decrease in temperature sensed by the sensor until a steady state condition is reached. Therefore, the power supply controller provides full power to the thermoelectric module for maximum cooling and down to some fraction of this as the temperature of the enclosure drops and only enough to counter thermal leakage once set point temperature is achieved. However, this variable power drive still results in excess power consumption and does not effectively control the temperature differential (ΔT) across the hot and cold plates of the thermoelectric module when exterior temperature at the fin stade are high.
Prior art devices have addressed the problem of heat transfer to and from thermoelectric modules by respective heat pipes by using common working fluid evaporator or condenser volumes to interface with a grouping of modules. The inherently unequal distribution and inefficient fluid flow characteristics cause unequal module load distribution as a basic problem in such a configuration. In addition, since heat pipes commercially available only as closed end tubes, manufacturing costs of such a configuration are excessive for commercial applications. This is especially true if the heat pipes are of the wicked and cored type, as are desirable for this application. Osmotic or mechanically pumped heat pipes introduce added complexity and expense to a device. Loop configuration heat pipes will have thermal gradients from top to bottom, inasmuch as this is the mechanism used to cause the fluid to rise in one arm of the loop and fall in the other. In this application, thermal gradients may cause thermal stress and unequal sharing of heat pumping loads in the modules. Basic open thermo-syphon configuration, without core or wicking, are low efficiency devices because of liquid pooling and thermal resistance effects in the fluid itself. Another problem is that as the fluid evaporates, it forms bubbles on the walls of the evaporator section that insulate the wall from the fluid. At the condensing end of a thermo-syphon, as the fluid becomes a liquid, the droplets interfere with contact of the vapor to the wall, again reducing efficiency. Any increase of the amount of heat energy to be transferred increases the magnitude of the problems in a thermo-syphon.
It is well known that as heat is displaced across the thermoelectric module this will cause a rise in temperature across the cold and hot plates of the module and this degrades the ability of the module to pump heat. The heat sinks connected to the cold and hot plates also build a thermal resistance and results in a significant temperature differential between the cold and hot plate. There is therefore a need to effectively manage the temperature differential across the thermoelectric module to increase the efficiency and life thereof as well as its power consumption.
The use of thermoelectric cooling modules in the construction of refrigerated enclosures has advantages and inconveniences. One advantage of these thermoelectric modules is that they do not use compressors and refrigerant conduits and associated devices which occupy a large space and which are noisy and often require maintenance. However, thermoelectric cooling modules have various inconveniences in that they are less efficient than conventional refrigeration systems using compressors and they are more expensive. Thermoelectric modules are also difficult to modulate by using a pulse width modulated (PWM) supply. It is also difficult to transfer heat quickly from an enclosure intended to be refrigerated.